The extraction of niobium (Nb) from ferroniobium (FeNb) has, in the past, been done in a variety of ways. The high cost of recovering niobium by the use of several of the known processes has deterred their commercial adoption.
Exemplary of the known methods include the extraction of niobium oxide (Nb.sub.2 O.sub.5) from FeNb in a multiple step process. The niobium oxide can then be metallothermically or carbothermically reduced to yield metal suitable for further purification by melting.
The state of the art extraction processes include chlorinating FeNb directly to produce ferric chloride (FeCl.sub.3) and niobium pentachloride (NbCl.sub.5) by passing chlorine through a bed of FeNb held at a temperature of from about 500.degree. C. to 1000.degree. C.
The reaction can be characterized as follows:
FeNb+4Cl.sub.2 .fwdarw.FeCl.sub.3 +NbCl.sub.5 +heat PA1 3Nb.sub.2 O.sub.5 +10Al.fwdarw.6Nb+5Al.sub.2 O.sub.3 +heat
This reaction is exothermic and once started provides considerable heat and must, therefore, be carefully controlled. The FeCl.sub.3 and NbCl.sub.5 produced must be separated and this is accomplished by passing the chlorides in the vapor state through a heated bed of sodium chloride (NaCl) where the FeCl.sub.3 forms a eutectic composition with the NaCl and is thereby removed from the vapor process stream. The NbCl.sub.5 can then be subsequently condensed by cooling.
This chlorination step utilizes toxic chlorine gas reacted exothermically at elevated temperatures and pressures. These conditions can produce severe corrosion and thereby potential safety problems. Special equipment is necessary for handling the highly pressurized, corrosive liquid chlorine and it must be safely vaporized, metered and fed into the reactor. Likewise, the most suitable material for reactor construction is graphite. This is a brittle material which can fracture and fail abruptly after a short time in use in this environment. Further, the chlorine is normally used in excess to ensure complete reaction with the FeNb and the excess must be neutralized creating an expensive, undesirable by-product.
The condensed NbCl.sub.5 can, if desired, be distilled to achieve higher purity material. Distilled or undistilled, NbCl.sub.5 is then hydrolyzed by its addition to water and then the bath is neutralized, and the insoluble product can optionally be dried before being calcined in a heated kiln in the presence of oxygen to produce Nb.sub.2 O.sub.5. The hydrolysis and neutralization steps can produce undesirable by-products and the drying and calcining steps are both energy intensive and expensive.
The Nb.sub.2 O.sub.5 obtained as described can then be metallothermically reduced with aluminum powder in a batch reaction to form Nb metal according to the following equation:
This reaction is very exothermic attaining temperatures in excess of the melting point of the products which are then separated by gravity while in the molten state. While expensive, metallothermic reduction is effective with good yields.
The other methods for Nb extraction from FeNb involve caustic or carbonate fusions, which when leached or washed, give niobium oxide which is fairly pure and may be further purified by chlorination or other means presently known to the art. Ultimately, the oxide must be metallothermically reduced as previously described, or carbothermically reduced to Nb metal.
The process of reducing Nb.sub.2 O.sub.5 carbothermically is difficult to do on a production basis since doing so requires large thermal input, vacuum vessels, and a careful balance of carbon to oxygen so that the resulting metal is not contaminated with either carbon or oxygen. If the carbon to oxygen ratio is maintained at nearly stoichiometric amounts, then the reaction proceeds rapidly until only a few percent of either remains unreacted. The reaction then proceeds slowly and it is difficult for it to reach completion. For this reason, carbothermic reduction is not currently used commercially.
Another method for extracting Nb from FeNb could theoretically be the direct electron beam melting and purification of FeNb by preferential vaporization of the Fe. This would be very expensive in practice as the melting point of FeNb is low and a great amount of electrical power is needed to superheat and vaporize the 5 to 40 weight percent of iron present. Though possible, it is not believed to be economically feasible.